The increasing sophistication of today's semiconductor chips requires corresponding technological advancements in the packaging of such chips. Ceramic chip carriers often make use of alumina-based substrates upon which discrete areas of multilayer metallization have been bonded. Generally, the metallization comprises (a) a base metallization layer bonded to the ceramic substrate, (b) a layer of nickel bonded to the base layer, and (c) a layer of gold bonded to the nickel layer. The base metallization layer is often formed of a refractory metal such as tungsten which may be screen-printed onto the substrate surface.
The nickel layer enhances wire bonding while also providing a good thermal expansion match between the tungsten and gold layers. While this layer may be applied by either electrolytic or electroless plating techniques, electroless plating is increasingly being used because of its ability to apply very uniform layers of nickel to complex, nonplanar surfaces, such as chip carrier surfaces having patterned contact holes and vias for electrical interconnection.
In these applications, it is often desirable that the nickel films be chemically pure. Unfortunately, the most common electroless nickel plating baths known in the art, which employ hypophosphite, borohydride or amine boranes as the chemical reducing agent, all deposit nickel films that contain about 1%-15% by weight phosphorous or boron as impurities. These impurities reduce the electrical conductivity of the nickel film; make it brittle; and cause it to be a relatively inferior surface for soldering.
It is known from the prior art that pure nickel films can be deposited from electroless plating baths in which hydrazine is used as the reducing agent. An exemplary electroless nickel plating bath of this kind is disclosed in W-D Haack's U.S. Pat. No. 3,915,716, and includes hydrazine, ammonia, monoethanolamine or diphosphate as a complexing agent, and carbonates or orthophosphates to buffer the bath at a pH between 11 and 12. Furthermore, in U.S. Pat. No. 3,198,659 and in "Thin Nickel Films by Hydrazine Autocatalytic Reduction", Electrochemical Technology, 1, 38-42 (1963), D. J. Levy discloses nickel plating compositions which include a nickel salt, sodium hydroxide, one of several complexing agents, and hydrazine as a reducing agent. In "Thick Nickel Deposits of High Purity by Electroless Methods", Plating, 54, 385-390 (1967), J. Dini et al. disclose a nickel plating composition which can contain nickel acetate, glycolic acid, tetrasodium EDTA, and hydrazine. V. M. Gershov et al. disclose a nickel-plating bath containing nickel sulphate, hydrazine sulphate and monoethanolamine, in Temperature Activation of Chemical Nickel- Plating in Hydrazine Solutions, Russian Engineering Journal, Volume 53, No. 10, pp. 73-74.
Unfortunately, the attributes of these electroless plating compositions are accompanied by several disadvantages. For example, the use of these baths to plate refractory metals often requires activation of the metal surface, such activation generally preceded by a series of complicated, rigorous cleaning steps.
Furthermore, some of the electroless plating baths of the prior art are highly unstable under temperature conditions necessary for plating nickel onto various substrates.
Moreover, the plating rates achieved by using some of these baths is very low, less than about 3 microns per hour, even at plating temperatures as high as 95.degree. C. Such plating rates, along with the ability to form only very thin nickel films, greatly diminishes the value of such baths in many commercial applications.
Other electroless plating compositions, such as those disclosed in the Gershov et al. reference mentioned above, are able to achieve high plating rates only when used at very high temperatures (100.degree. C.-200.degree. C.).
It is therefore an object of the present invention to provide a highly stable electroless nickel plating composition.
It is another object to provide a plating composition which can be used to apply chemically pure nickel to a metal substrate.
It is a further object to provide a nickel plating composition which allows nickel to be plated directly upon refractory metal surfaces without prior activation of the surfaces.
It is still another object of the present invention to provide a reliable and practical method of electrolessly applying nickel to a metal purface at high plating rates and at moderate plating bath temperatures.
Another object of the present invention is to provide a method for preparing such an electroless nickel plating composition.